CS

E 1

23b

CS

E 1

23b

Com

mun

icat

ions

Sof

twar

eC

omm

unic

atio

ns S

oftw

are

Spr

ing

2002

Spr

ing

2002

Lect

ure

3: R

elia

ble

Com

mun

icat

ions

Lect

ure

3: R

elia

ble

Com

mun

icat

ions

Ste

fan

Sav

age

Ste

fan

Sav

age So

me

slid

es c

oure

sty

Dav

id W

ethe

rall

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on2

Adm

inis

trat

iva

Adm

inis

trat

iva

Hom

e pa

ge is

up

and

wor

king

◆ht

tp://

ww

w-c

se.u

csd.

edu/

clas

ses/

sp02

/cse

123B

/◆

Cla

ss n

otes

incl

uded

◆S

ign

up fo

r the

cla

ss m

ailin

g lis

t (in

stru

ctio

ns o

n th

e W

eb p

age)

Firs

t hom

ewor

k w

ill b

e as

sign

ed n

ext T

uesd

ay

Cla

ss o

n Th

ursd

ay is

***

canc

elle

d***

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on3

Last

Tim

eLa

st T

ime

We

talk

ed a

bout

the

netw

ork

laye

r (IP

) and

in

tern

etw

orki

ng.

We

assu

me:

the

netw

ork

prov

ides

bes

t-effo

rt (i.

e. u

nrel

iabl

e) d

eliv

ery

of p

acke

ts fr

om o

ne

host

to a

noth

er◆

How

that

is d

one,

rout

ing,

is le

ft fo

r a fu

ture

cla

ss

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on4

Tod

ay’s

cla

ssT

oday

’s c

lass

We

begi

n on

the

trans

port

laye

r◆

Bui

lds

on th

e se

rvic

es o

f the

Net

wor

k la

yer

◆C

omm

unic

atio

n be

twee

n pr

oces

ses

runn

ing

on

host

sP

rinci

ple

focu

s◆

How

do

we

ensu

re th

at a

mes

sage

is re

liabl

yco

mm

unic

ated

from

one

hos

t to

anot

her?

Topi

cs◆

AR

Q◆

Slid

ing

win

dow

s◆

Ret

rans

mis

sion

tim

ers

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on5

Tho

ught

exp

erim

ent

Tho

ught

exp

erim

ent

You

wan

t to

send

a lo

ng le

tter t

o yo

ur fr

iend

A

ll yo

u ha

ve (a

nd a

ll yo

ur fr

iend

has

) is

post

card

sP

ostc

ards

get

lost

in th

e m

ail,

dela

yed,

da

mag

edH

ow d

o yo

u se

nd th

e le

tter?

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on6

Rel

iabl

e T

rans

mis

sion

Rel

iabl

e T

rans

mis

sion

How

do

we

relia

bly

send

a m

essa

ge w

hen

pack

ets

can

be lo

st in

the

netw

ork?

Som

e op

tions

◆D

etec

t a lo

ss a

nd re

trans

mit

(Cer

f&K

ahn7

4, a

nd

othe

rs)

◆S

end

redu

ndan

tly (B

yers

et a

l.98,

and

oth

ers)

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on7

Aut

omat

ic R

epea

t R

eque

st

Aut

omat

ic R

epea

t R

eque

st

(AR

Q)

(AR

Q)

Pac

kets

can

be

corr

upte

d or

lost

. How

do

we

add

relia

bilit

y?A

ckno

wle

dgm

ents

(AC

Ks)

and

retra

nsm

issi

ons

afte

r a

timeo

utA

RQ

is g

ener

ic n

ame

for p

roto

cols

bas

ed o

n th

is

stra

tegy

Sen

der

Rec

eive

r

Dat

a ACK

Timeout

Time

Sen

der

Rec

eive

r

Dat

a

Timeout

Dat

a

ACK

Timeout

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on8

The

Nee

d fo

r S

eque

nce

The

Nee

d fo

r S

eque

nce

Num

bers

Num

bers

Sen

der

Rec

eive

rS

ende

rR

ecei

ver

Fram

e

ACK

imeout

Dat

a

ACK

Timeout

Dat

a ACK

imeout

Dat

a

ACK

TimeoutT

T

In th

e ca

se o

f AC

K lo

ss (o

r poo

r cho

ice

of

timeo

ut) t

he re

ceiv

er c

an’t

dist

ingu

ish

this

m

essa

ge fr

om th

e ne

xt◆

Nee

d to

und

erst

and

how

man

y pa

cket

s ca

n be

ou

tsta

ndin

g an

d nu

mbe

r the

pac

kets

; her

e, a

sin

gle

bit w

ill d

o

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on9

Sto

pS

top

-- and

and

-- Wai

tW

ait

Onl

y on

e ou

tsta

ndin

g pa

cket

at

a ti

me

Als

o ca

lled

alte

rnat

ing

bit

prot

ocol

0 1 0 1

Sen

der

Rec

eive

r

0 1 10

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on10

How

doe

s re

ceiv

er r

ecog

nize

H

ow d

oes

rece

iver

rec

ogni

ze

a du

plic

ate?

a du

plic

ate?

Seq

uenc

e #

in p

acke

t is

finite

How

man

y bi

ts d

o w

e ne

ed?

◆O

ne b

it fo

r sto

p an

d w

ait

◆W

on’t

send

seq

#1 u

ntil

rece

ive

AC

K

for s

eq#0

◆O

nly

allo

ws

one

pack

et in

flig

ht

Pkt 0

ACK 0

Timeout

Pkt 0

AC

K 1

1Pk

tA

CK

0

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on11

Wha

t if

pac

kets

are

del

ayed

?W

hat

if p

acke

ts a

re d

elay

ed?

Nev

er re

use

a se

q#?

Fi

nite

…R

equi

re in

ord

er d

eliv

ery?

Pre

vent

ver

y la

te d

eliv

ery?

◆TT

L: D

ecre

men

t hop

cou

nt

per p

acke

t, di

scar

d if

exce

eded

◆S

eq#s

not

reus

ed w

ithin

de

lay

boun

dTr

ust i

ssue

s?

0 110 0Accept!

Reject!

0

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on12

Wha

t ha

ppen

s if

a m

achi

ne

Wha

t ha

ppen

s if

a m

achi

ne

cras

hes?

cras

hes?

How

do

we

dist

ingu

ish

pack

ets

sent

bef

ore

and

afte

r reb

oot?

Whi

ch s

eq#

to u

se?

Sol

utio

ns◆

Res

tart

sequ

ence

# a

t 0?

◆A

ssum

e bo

ot ta

kes

max

pac

ket d

elay

?◆

Cho

ose

seq

# at

rand

om a

nd h

ope?

◆U

se s

tabl

e st

orag

e an

d in

crem

ent h

igh

orde

r bits

of

seq

# on

eve

ry b

oot

Rea

lity:

Peo

ple

don’

t wor

ry a

bout

this

◆S

low

rebo

ots,

exp

licit

conn

ectio

n m

anag

emen

t, to

lera

nt u

sers

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on13

Lim

itat

ions

of

Sto

pLi

mit

atio

ns o

f S

top

-- and

and

-- Wai

tW

ait

Lous

y pe

rform

ance

if w

ire ti

me

<< p

rop.

del

ay◆

How

bad

? W

ant t

o ut

ilize

all

avai

labl

e ba

ndw

idth

◆N

eed

to k

eep

mor

e da

ta “i

n fli

ght”

◆H

ow m

uch?

Rem

embe

r the

ban

dwid

th-d

elay

pr

oduc

t?A

lso

limite

d by

qua

lity

of ti

meo

ut (h

ow lo

ng?)

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on14

Pip

elin

ed t

rans

mis

sion

Pip

elin

ed t

rans

mis

sion

Sen

d m

ultip

le p

acke

ts w

ithou

t w

aitin

g fo

r the

firs

t to

be A

CK

edR

elia

ble,

uno

rder

ed d

eliv

ery:

◆S

end

new

pac

ket a

fter e

ach

AC

K◆

Sen

der k

eeps

list

of u

nAC

K’e

dpa

cket

s an

d re

send

s af

ter t

imeo

ut◆

Rec

eive

r sam

e as

sto

p &

wai

tP

rob:

Wha

t if p

acke

t 2 k

eeps

be

ing

lost

?◆

Rec

eive

r mus

t buf

fer a

ll pa

cket

s af

ter 2

◆P

oten

tial b

uffe

r ove

rflow

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on15

Slid

ing

Win

dow

S

lidin

g W

indo

w ––

Sen

der

Sen

der

Win

dow

bou

nds

outs

tand

ing

data

◆Im

plie

s ne

ed fo

r buf

ferin

g at

sen

der

“Las

t” A

CK

app

lies

to in

-ord

er d

ata

Wha

t to

do o

n a

timeo

ut?

◆G

o-B

ack-

N: o

ne ti

mer

, sen

d al

l una

ckno

wle

dged

da

ta o

n tim

eout

◆S

elec

tive

Rep

eat:

timer

per

pac

ket,

rese

nd a

s ne

eded

≤W

indo

w S

ize

“Las

t” A

CK

Last

Sen

t

……

Sen

der:

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on16

Slid

ing

Win

dow

S

lidin

g W

indo

w ––

Rec

eive

rR

ecei

ver

Rec

eive

r buf

fers

too:

◆da

ta m

ay a

rriv

e ou

t-of-o

rder

◆or

fast

er th

an c

an b

e co

nsum

ed (f

low

con

trol)

Rec

eive

r AC

K c

hoic

es:

◆In

divi

dual

, Cum

ulat

ive

(TC

P),

Sel

ectiv

e (n

ewer

TC

P),

Neg

ativ

e

≤R

ecei

ve W

indo

w

“Las

t” R

ecei

ved

Larg

est A

ccep

ted

……

Rec

eive

r:

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on17

Slid

ing

Win

dow

S

lidin

g W

indo

w ––

Tim

elin

eT

imel

ine

Send

erR

ecei

ver

……

Time

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on18

Slid

ing

Win

dow

Fun

ctio

nsS

lidin

g W

indo

w F

unct

ions

Slid

ing

win

dow

is a

mec

hani

smIt

supp

orts

mul

tiple

func

tions

:

Rel

iabl

e de

liver

yIn

-ord

er d

eliv

ery

Flow

con

trol

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on19

Dec

idin

g W

hen

to R

etra

nsm

itD

ecid

ing

Whe

n to

Ret

rans

mit

How

do

you

know

whe

n a

pack

et h

as b

een

lost

?◆

Ulti

mat

ely

send

er u

ses

timer

s to

dec

ide

whe

n to

re

trans

mit

But

how

long

sho

uld

the

timer

be?

◆To

o lo

ng: i

neffi

cien

t (la

rge

dela

ys, p

oor u

se o

f ba

ndw

idth

)◆

Too

shor

t: m

ay re

trans

mit

unne

cess

arily

(cau

sing

ext

ra

traffi

c)R

ight

tim

er is

bas

ed o

n th

e ro

und

trip

time

(RTT

)◆

Whi

ch v

arie

s gr

eatly

(pat

h le

ngth

and

que

uing

)

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on

A S

impl

e N

etw

ork

Mod

elA

Sim

ple

Net

wor

k M

odel

Buf

fers

at r

oute

rs u

sed

to a

bsor

b bu

rsts

whe

n in

put

rate

> o

utpu

tLo

ss (d

rops

) occ

ur w

hen

send

ing

rate

is p

ersi

sten

tly >

dr

ain

rate

Des

tinat

ion

1.5-

Mbp

s T1

link

Rou

ter

Sou

rce

2

Sou

rce

1

100-

Mbp

s FD

DI

10-M

bps

Ethe

rnet

Pac

kets

que

ued

here

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on21

Eff

ects

of

Ear

ly

Eff

ects

of

Ear

ly

Ret

rans

mis

sion

sR

etra

nsm

issi

ons

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on22

Con

gest

ion

Col

laps

eC

onge

stio

n C

olla

pse

In th

e lim

it, e

arly

retra

nsm

issi

ons

lead

to

cong

estio

n co

llaps

e◆

Sen

ding

mor

e pa

cket

s in

to th

e ne

twor

k w

hen

it is

ov

erlo

aded

exa

cerb

ates

the

prob

lem

of c

onge

stio

n◆

Net

wor

k st

ays

busy

but

ver

y lit

tle u

sefu

l wor

k is

be

ing

done

This

hap

pene

d in

real

life

~19

87◆

Led

to V

an J

acob

son’

s TC

P a

lgor

ithm

s, w

hich

form

th

e ba

sis

of c

onge

stio

n co

ntro

l in

the

Inte

rnet

toda

y[S

ee “C

onge

stio

n A

void

ance

and

Con

trol”,

S

IGC

OM

M’8

8]

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on23

Est

imat

ing

Est

imat

ing

RT

Ts

RT

Ts

Idea

: Ada

pt b

ased

on

rece

nt p

ast m

easu

rem

ents

Sim

ple

algo

rithm

:◆

For e

ach

pack

et, n

ote

time

sent

and

tim

e ac

kre

ceiv

ed◆

Com

pute

RTT

sam

ples

and

ave

rage

rece

nt s

ampl

es fo

r tim

eout

◆E

stim

ated

RTT

= α

x E

stim

ated

RTT

+ (1

-α

) x S

ampl

eRTT

◆Th

is is

an

expo

nent

ially

-wei

ghte

d m

ovin

g av

erag

e th

at

smoo

thes

the

sam

ples

. Typ

ical

ly, α

= 0.

8 to

0.9

.◆

Set

tim

eout

to s

mal

l mul

tiple

(2) o

f the

est

imat

e to

cap

ture

va

riatio

n ar

ound

mea

n.

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on24

Est

imat

ed R

etra

nsm

it T

imer

Est

imat

ed R

etra

nsm

it T

imer

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on25

Kar

nK

arn

/Par

trid

ge A

lgor

ithm

/Par

trid

ge A

lgor

ithm

Pro

blem

: RTT

for r

etra

nsm

itted

pac

kets

am

bigu

ous

Sol

utio

n: D

on’t

mea

sure

RTT

for r

etra

nsm

itted

pac

kets

an

d do

not

rela

x ba

cked

of t

imeo

ut u

ntil

valid

RTT

m

easu

rem

ents

Sen

der

Rec

eive

r

Orig

inal

tran

smis

sion

ACK

SampleRTT

Ret

rans

mis

sion

Sen

der

Rec

eive

r

Orig

inal

tran

smis

sion

ACK

SampleRTT

Ret

rans

mis

sion

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on26

Jaco

bson

/Ja

cobs

on/ K

arel

sK

arel

sA

lgor

ithm

Alg

orit

hmP

robl

em:

◆V

aria

nce

in R

TTs

gets

larg

e as

net

wor

k ge

ts lo

aded

◆S

o an

ave

rage

RTT

isn’

t a g

ood

pred

icto

r whe

n w

e ne

ed it

m

ost

Sol

utio

n: T

rack

var

ianc

e to

o.◆

Diff

eren

ce =

Sam

pleR

TT–

Est

imat

edR

TT◆

Est

imat

edR

TT=

Est

imat

edR

TT+

(δx

Diff

eren

ce)

◆D

evia

tion

= D

evia

tion

+ δ(

|Diff

eren

ce|-

Dev

iatio

n)◆

Tim

eout

= µ

x E

stim

ated

RTT

+ φ

x D

evia

tion

◆In

pra

ctic

e, δ

= 1/

8, µ

= 1

and φ

= 4

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on27

Est

imat

e w

ith

Mea

n +

Est

imat

e w

ith

Mea

n +

Var

ianc

eV

aria

nce

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on28

Can

we

shor

tcut

the

tim

eout

?C

an w

e sh

ortc

ut t

he t

imeo

ut?

Pro

blem

◆If

a pa

cket

is d

ropp

ed in

the

netw

ork,

the

send

er

has

to w

ait u

ntil

timeo

ut o

ccur

s be

fore

reac

ting

If pa

cket

s ar

e us

ually

in o

rder

then

out

-of-r

der

pack

ets

impl

y th

at a

pac

ket w

as lo

st◆

Neg

ativ

e A

CK

»R

ecei

ver r

eque

sts

mis

sing

pac

ket

◆Fa

st re

trans

mit

»R

ecei

ver A

CK

sou

t-of-o

rder

pac

kets

with

seq

# of

last

co

ntig

uous

pac

ket

»W

hen

send

er re

ceiv

es m

ultip

le d

uplic

ate

ackn

owle

dgem

ents

rese

nds

mis

sing

pac

ket

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on29

Fast

ret

rans

mit

Fast

ret

rans

mit

Send

erR

ecei

ver

Time

……1 2 3

1 1

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on30

Alt

erna

tive

s to

ret

rans

mis

sion

?A

lter

nati

ves

to r

etra

nsm

issi

on?

Red

unda

ncy

◆S

end

addi

tiona

l dat

a to

com

pens

ate

for l

ost

pack

ets

Why

not

use

retra

nsm

issi

on◆

Mul

ticas

t»

Lots

of r

ecei

vers

If ea

ch o

ne A

CK

/NA

K th

en h

ard

to s

cale

Lots

of m

essa

ges

Lots

of s

tate

»H

eter

ogen

eous

rece

iver

sM

odem

vs

100M

Bps

con

nect

ed h

osts

◆O

ne-w

ay o

r ver

y lo

ng d

elay

cha

nnel

s (s

pace

craf

t)

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on31

Sim

ples

t ve

rsio

nS

impl

est

vers

ion

Sen

d ev

ery

pack

et tw

ice

Mus

t los

e bo

th p

acke

ts in

a p

air t

o pr

even

t m

essa

ge fr

om b

eing

del

iver

ed

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on32

Gen

eral

izat

ion:

G

ener

aliz

atio

n:

Forw

ard

Err

or C

orre

ctio

n (F

EC

)Fo

rwar

d E

rror

Cor

rect

ion

(FE

C)

Use

era

sure

cod

es to

redu

ndan

tly e

ncod

e k

sour

ce p

acke

ts in

to k

*m e

ncod

ed p

acke

ts◆

Ree

d S

olom

on C

odes

◆To

rnad

o co

des

Mul

ticas

t/bro

adca

st e

ncod

ed p

acke

ts

cont

inua

llyA

ny re

ceiv

er c

an re

cons

truct

mes

sage

from

an

y k

pack

ets

in th

e se

t of k

*m

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on33

Som

etim

es r

efer

red

to a

s a

Som

etim

es r

efer

red

to a

s a

“Dig

ital

Fou

ntai

n”“D

igit

al F

ount

ain”

Use

r 1U

ser 2

Tran

smis

sion

5 ho

urs

4 ho

urs

3 ho

urs

2 ho

urs

1 ho

ur

0 ho

urs

File

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on34

Pro

s an

d C

ons

of F

orw

ard

Pro

s an

d C

ons

of F

orw

ard

Err

or C

orre

ctio

nE

rror

Cor

rect

ion

Pro

◆E

very

pac

ket c

an b

e us

eful

for a

ll cl

ient

s

◆W

ell s

uite

d to

mul

ticas

t situ

atio

n

Con

◆S

ends

mor

e da

ta th

an id

eally

nec

essa

ry

◆N

eed

larg

e bl

ock

size

s fo

r effi

cien

cy

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on35

Sum

mar

yS

umm

ary

Tran

spor

t lay

er a

llow

s pr

oces

ses

to

com

mun

icat

e w

ith s

trong

er g

uara

ntee

s, e

.g.,

relia

bilit

yR

elia

bilit

y m

echa

nism

s◆

AR

Q»

Slid

ing

Win

dow

+ re

trans

mis

sion

for e

ffici

ency

»R

etra

nsm

issi

on ti

mer

mus

t be

adap

tive

◆FE

C»

In re

stric

ted

setti

ngs

April

9, 2

002

CSE

123

b --

Lect

ure

3 –

Rel

iabl

e Tr

ansm

issi

on36

For

next

tim

e…Fo

r ne

xt t

ime…

Rem

embe

r tha

t nex

t tim

e is

a w

eek

from

toda

y

Rea

d C

h 6.

3-6.

4